Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power.
The turbocharger shaft is supported by a central bearing housing disposed between the turbine and compressor wheels. The turbocharger shaft rotates within a bearing cartridge housed in an axial bore of the bearing housing.
The bearing cartridge has inner and outer races radially separated by bearing elements in the form of rollers, ball bearings, etc. The outer race is received in the axial bore of the bearing housing and is fixed against rotation, typically by means of a pin. The inner race is fixed to the turbocharger shaft by an interference fit.
In order to free the bearing cartridge, during disassembly of the turbocharger, the turbocharger shaft and the turbine wheel must first be removed from the bearing housing, specifically by pulling the shaft through the axial bore in the bearing housing.
Due to the interference fit of the shaft and bearing cartridge, this tends to pull the inner race with the shaft and so strip the inner race away from the rest of the bearing cartridge, resulting in damage to the cartridge. It is considerably expensive to replace a damaged bearing cartridge.
The same problem occurs with bearing assemblies that do not comprise a bearing cartridge but simply have inner and outer races radially separated by bearing elements.
A further problem is that some bearing assemblies, for example rolling element bearing (REB) assemblies, need to be preloaded in order to function and take any thrust loading. The compressor wheel is axially fixed along the turbocharger shaft by a compressor nut received on a threaded end of the shaft that extends beyond a hub of the compressor wheel. Preloading of the bearing elements is obtained by tightening the compressor nut on the shaft.
However, tightening the compressor nut also produces a tensile axial force on the turbocharger shaft, which acts to extend the shaft. This can result in increased axial slip of the compressor wheel during operation of the turbocharger. This can also result in rotation of the compressor wheel relative to the shaft, as the shaft rotates, resulting in a loss of useful work done and thereby a reduction in efficiency.
Whereas the turbine of a turbocharger drives a compressor, in a power turbine the end of the turbine shaft remote from the turbine wheel transmits power via a mechanical coupling. In a turbocompound engine two turbines are provide in series, with a power turbine connected in series with the turbine of a turbocharger. The power turbine is used to generate additional power and a gear wheel may be fixed to the end of the power turbine shaft in order to transmit that power to the crankshaft of the engine via an appropriate coupling (such as for example a fluid coupling or a gear or other drive mechanism), hydraulically, mechanically or electrically. As with a turbocharger, the shaft of a power turbine is supported on bearing assemblies. The bearing arrangement at the turbine end of the shaft may be substantially the same as that found in a turbocharger, although the bearing arrangement at the drive end of the shaft may be a ball bearing assembly.